Memory can be divided into two types; volatile and non-volatile. Volatile memory includes static random access memory (SRAM) and dynamic random access memory (DRAM). An advantage of volatile memory is that it permits random access. A disadvantage of volatile memory is that data is lost when power is lost.
Non-volatile memory may include mask read only memory (ROM), electrically erasable programmable read only memory (EEPROM), and flash memory. Non-volatile memory does not lose data when power is lost, but generally does not permit random access and is slower than volatile memory.
There are several candidates for non-volatile random access memory that are higher speed and lower cost. These candidates include ferroelectric random access memory (FRAM), magnetic random access memory (MRAM) and phase changeable random access memory (PRAM).
A PRAM is a memory device using a phase changeable material to store a bit of data. Phase changeable materials that are used in memory devices may exhibit at least two different states. These states may be referred to as the amorphous and crystalline states. These states may be distinguished because the amorphous state generally exhibits higher resistivity than the crystalline state. Generally, the amorphous state involves a more disordered atomic structure.
A compound material of germanium (Ge), antimony or stibium (Sb) and tellurium (Te), also known as GST, may be used as the phase changeable material. An example of a conventional PRAM, including a silicon substrate 10, a bottom insulation 13, a bottom electrode 15, the phase changeable material (PCM) 17, a top electrode 19, an insulating interlayer 21, and a top interconnection 23, is shown in FIG. 1. When the top interconnection 23 is biased with a voltage, current flows from the top electrode 19 to the bottom electrode 15 through the PCM 17. The current generates joule heat from the bottom electrode 15. This heat influences the crystalline state of the PCM 17. If the heat is at a higher temperature for a shorter time period, a portion of the PCM 17 is in an amorphous state with a higher resistance. If the heat is at a lower temperature for a longer time period, a portion of the PCM 17 is in a crystalline state with a lower resistance.
There are several conventional PRAMs, some of which use a CMOS transistor as the cell access device and “push” power from a top electrode to a column line through a cell transistor. Others use a common PCM and “push” power from a top electrode to collector node of cell bipolar transistor. Still others use a common PCM and a bipolar diode for the cell access device.